1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically include a plurality of base stations or access points that provide wireless connectivity to mobile units within a geographical area. The device that provides the wireless connectivity and the geographic area are both conventionally referred to as a cell. The air interface between the base station or access point and the mobile unit supports one or more downlink (or forward link) channels from the base station to the mobile unit and one or more uplink (or reverse link) channels from the mobile units to the base station. The uplink and/or downlink channels include traffic channels, signaling channels, broadcast channels, paging channels, pilot channels, and the like. The channels can be defined according to various protocols including time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA), orthogonal frequency division multiple access (OFDMA), as well as combinations of these techniques. The geographical extent of each cell may be time variable and may be determined by the transmission powers used by the base stations, access point, and/or mobile units, as well as by environmental conditions, physical obstructions, and the like.
Conventional hierarchical wireless communication systems include a central element such as a Radio Network Controller (RNC) or a Base Station Controller (BSC). The central controller coordinates operation of the base stations and performs radio resource control functions such as call admission and resource allocation. For example, when data is available for a target mobile unit, the RNC may transmit paging messages to the target mobile unit via one or more base stations or node-Bs. The target mobile unit may establish a wireless link to one or more of the base stations in response to receiving the page from the wireless communication system. A radio resource management function within the RNC coordinates/allocates the resources used by the base stations and/or the target mobile unit for communication over the air interface. The radio resource management function can also perform fine grain control to allocate and release resources for broadcast transmission over a set of base stations.
One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of access points, such as base station routers or eNodeBs (eNBs), which implement distributed communication network functionality. For example, each base station router or eNB may combine RNC/BSC and/or packet data serving node (PDSN) functions in a single entity that manages radio links between one or more mobile units and an outside network, such as the Internet. Base station routers and eNBs wholly encapsulate the cellular access technology and may proxy functionality that utilizes core network element support to provide equivalent IP functions. For example, IP anchoring in a UMTS base station router may be offered through a Mobile IP Home Agent (HA) and the GGSN anchoring functions that the base station router proxies through equivalent Mobile IP signaling. Compared to hierarchical networks that use centralized control, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points, e.g. base station routers and/or eNBs, to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the separate RNC and PDSN entities in hierarchical networks may be reduced or removed.
In normal operation, base stations (or base station routers) serving each cell radiate a signal such as a pilot signal and mobile units in the corresponding cell can detect the presence of the base station by detecting the pilot signal. Mobile units can access the wireless communication system by establishing communication links over the air interface with cells that have a sufficiently strong pilot signals. If the mobile unit and the cell establish the connection, the cell becomes the termination point for the wireless communication link and the mobile unit can register with the cell to begin transmitting and/or receiving traffic over the air interface. Consequently, the number of attempted accesses, the number of terminations, the number of registrations, and the amount of traffic supported by a normally operating cell is roughly proportional to the number of mobile units that are located in each cell. However, cells do not always operate as expected.
One example of a cell that is not operating correctly is a “sleeping cell.” A sleeping cell may be visible to mobile units in the cell because it is radiating the expected pilot signal but the sleeping cell may not be receiving the expected number of access requests, terminations, registrations, and/or traffic. In another case, the sleeping cell may not even be radiating and so may be invisible to mobile units in the cell. Since mobile units cannot see the sleeping cell, this cell may also be receiving a smaller than expected number of access requests, terminations, registrations, and/or traffic. Sleeping cells may result from hardware, firmware, and/or software problems in the base station. Regardless of the source of the problem, conventional base stations and access points do not include alarms to report sleeping cell behavior to the system. The network therefore interprets the sleeping cell as a coverage area hole, which can degrade coverage and/or capacity of the wireless communication system. User experience can also be degraded.